There is considerable interest in replacing fossil fuels with hydrogen because of hydrogen's high energy density per unit weight, its readily availability through the electrolysis of water, and the absence of polluting byproducts from its use. A number of technological components present challenges in making this transition to a hydrogen economy, and in the development of appropriate systems and infrastructure that can integrate into those that already exist. Particularly in the automotive industry, one significant challenge involves the current paucity of fully satisfactory systems for hydrogen storage, ones that are safe, reliable, conformable, light-weight, and comprehensively economic. The technical issues underlying various approaches to hydrogen storage include the form within which hydrogen is stored, the nature of the medium holding the hydrogen, and the operation of the containers holding the medium.
Holding hydrogen either as a compressed gas or as a cryogenic liquid are the currently standard forms of hydrogen storage. Compressed hydrogen can be stored in high pressure tanks (up to 10,000 p.s.i.). A problem with this method is that hydrogen diffuses very effectively, particularly when under high pressure, and currently available high pressure tanks do not effectively prevent such diffusion over an extended period of time. The requirement that tank materials be lightweight, and the fact that tank failure or damage in the event of an accident would be catastrophic provides further reason for pause. Storage of hydrogen in liquid form at cryogenic temperatures is also an approach that shows some promise. However, liquid hydrogen storage is burdened with a high boil-off rate that limits the dormancy of cryogenic liquid tanks, the time period for which a tank is stable without venting of accrued pressure.
Metal hydrides, such as magnesium-based alloys, have been used as media to store hydrogen. Although this method does not require high pressure and is operable at room temperature, there are numerous drawbacks. Metal hydrides are heavy, generally heavier than the hydrogen gas by a factor of about 50. Metal hydrides also undesirably contaminate the hydrogen as it is released. Further, metal hydride storage is not energy-efficient in this context; the energy required to extract the hydrogen from the metal hydride is equivalent to nearly half the amount stored within it. Finally, the rate of heat transfer within metal hydrides is limited by the fact that they are solids, and are not able to benefit from the higher rate of heat transfer afforded by gas that exists in porous materials. Metal hydride storage has been disclosed by Liu et al. (U.S. Pat. No. 4,358,316), by Bernauer et al. (U.S. Pat. No. 4,446,101), and by Ovshinsky et al. (U.S. Pat. No. 6,328,821).
Cryogenic surface adsorption of hydrogen on porous host media represents an alternative technological approach. Activated carbon, which has high surface area, has been used to store hydrogen at cryogenic temperatures and moderate pressures (50–70 bar), as has been described by Schwarz (U.S. Pat. No. 4,716,736). Cryogenic storage in activated carbon can be done at a 80K, a temperature higher than that required for liquid hydrogen storage. This relatively high operating temperature is helpful, and carbon is available in large supply; nevertheless an activated carbon solution to hydrogen storage is not free of technical problems. Commonly available activated carbon, for example, is not very pure, and contaminants are released with the hydrogen, and it is difficult to obtain release of all hydrogen stored on activated carbon. Other problems associated with activated carbon include low weight percent storage capacity and the need to maintain cryogenic temperatures. The pros and cons of the use of carbon are discussed by Hynek et al. in “Hydrogen storage by carbon sorption,” Int. J. Hydrogen Energy, 22, No. 6; 601–610, 1997.
Alternative forms of carbon for hydrogen storage that have been explored include carbon nanotubes and graphite fibers, which according to Rodriguez et al. (U.S. Pat. Nos. 5,653,951 and 6,159,538) bind hydrogen by chemisorption. Another approach using nano-structured media constructed from light elements, as described in pending patent applications of Bradley et al. (U.S. Application No. 60/020,392) and Kwon et al. (U.S. Application No. 60/020,344) is one in which hydrogen is adsorbed by physisorption. Other carbon-based approaches include the utilization of turbostratic microstructures, as described by Maeland (U.S. Pat. No. 6,290,753) and aerogel preparations of carbon fullerenes have been described by Lamb (U.S. Pat. No. 5,698,140). While structured forms of carbon do offer potential advantages over non-structured activated carbon, the basic thermodynamic properties of carbon which determine the operating temperatures at which hydrogen is desorbed from the medium remain substantially the same.
In the hydrogen economy, hydrogen will be stored in different places, in different unit volumes, and in operationally varying configurations, as it moves down the supply chain from producers to consumers. Producers will need to store large inventory volumes. Hydrogen will be stored in transporting vessels as it travels from producers to distributors. Fuel distributors, including stations that deliver fuel for vehicles, other power-driven devices, and electronic devices, will have large quantities on hand. Small point-of-use storage containers will also be required in micro-power plants, vehicles, and personal electronics. All these hydrogen storage applications have in common the need to maximize the amount of gas stored per unit tank volume, and differ fundamentally from the gasoline distribution system in which the fuel retains the same and incompressible form throughout the supply chain.
The requirements for hydrogen storage in the hydrogen economy have not yet been fully met, and there is a need for storage media that permit systems to operate at higher temperatures and lower pressures than those presently adapted for high pressure hydrogen storage. Further, there is a need for the development of surrounding systems that can accommodate and enable the integration of the storage media into all aspects of the hydrogen fuel distribution system, from the manufacturing source to the engines and devices in which the hydrogen fuel is converted into useful power.